public class trap { public int trap(int[] height) {
    int ans = 0;
    int left = 0;
    int right = height.length - 1;
    int leftMax = 0;
    int rightMax = 0;
    while(left < right){
        leftMax = Math.max(leftMax,height[left]);
        rightMax = Math.max(rightMax,height[right]);
        if(height[left] < height[right]){
            ans += leftMax - height[left];
            left++;
        }else{
            ans += rightMax - height[right];
            right--;
        }
    }
    return ans;
}
}
